PPP over GSM slowspeed backdoor to my Echolink node

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1 Background

I recently found that the link between my parents' house and the hut in which the device is located was broken. The reason was some weird misconfiguration of our house's LAN network. And since a sysop of an automatic station is legally required to be able to switch a transmitter off in case of troubles and interferences, I decided to set up this feature for my Echolink node.

2 Plan

  • In order to provide access to the device, I decided to implement a neat little dial-in solution using my Arduino Duemillanove and the GSM Playground board with a Telit GE863-Quad module. Since I have been rather successful with the Arduino up to now and since I now have an Arduino Mega, I can dispense with the Duemillanove and no longer keep it as experimental stuff.
  • The idea is to build the couple (Duemillanove + Playground) into my Echolink controll system, connected to the Intel Atom board through a USB2Serial connection which is the standard connection to the Arduino anyway. It is a rather comfortable interface because there's no handling of those incompatible voltage levels, and the mechanical construction questions are also solvend neatly because the Arduino has some kind of form factor which allows for easy integration. Well, it is not the smallest possible form of building up something like this, but never mind.
  • There's a little switch on the Playground board which allows to disable the Arduino output to the Telit module. This is rather helpful if you don't want to use the Arduino anyway, which is the case here, because I want to react on incoming PPP calls but also at the same time be able to use the Telit module as a SMS gateway for special status and alarm messages.
  • On the compuer side I will run a mgetty to provide for a ppp daemon and a self written program to interface with it for the incoming SMS messages over the Hamnet lines.
  • Since the lowest tray of the case is currently empty (It used to carry a harddisk but I crashed the disks several time because of the narrow specification of the voltage supply (which I failed to meet the expensive way several times). This case houses a 5V supply line and thus has everything in place.

3 Resources

  • Some Python code to generate PDU messages out of clear text. PDU messages are a representation form of an SMS which encodes every necessary bit of information in one line.
  • Arduino and GSM - here's a number of links also, concerning the boards involved. Also some important comments on the GSM library of HW Kitchen, which I decided not to use at all. It is good for the beginner and sure also for many other simple problems and scenarios, but it is more limiting to what I intend to do.
  • Once again the fantastic and very comprehensive AT Hayes commandset documentation for the Telit module: http://www.telit.com/module/infopool/download.php?id=542